HTTP authentication with PHP

The HTTP Authentication hooks in PHP are only available when it is
running as an Apache module and is hence not available in the CGI version.
In an Apache module PHP script, it is possible to use the
header() function to send an "Authentication Required"
message to the client browser causing it to pop up a Username/Password
input window. Once the user has filled in a username and a password,
the URL containing the PHP script will be called again with the
predefined variablesPHP_AUTH_USER, PHP_AUTH_PW,
and AUTH_TYPE set to the user name, password and
authentication type respectively. These predefined variables are found
in the $_SERVER and
$HTTP_SERVER_VARS arrays. Both "Basic" and "Digest"
(since PHP 5.1.0) authentication methods are supported. See the
header() function for more information.

Note:
PHP Version NoteSuperglobals,
such as $_SERVER, became
available in PHP » 4.1.0.
$HTTP_SERVER_VARS has been available since PHP 3.

An example script fragment which would force client authentication
on a page is as follows:

Note:
Compatibility Note
Please be careful when coding the HTTP header lines. In order to guarantee maximum
compatibility with all clients, the keyword "Basic" should be written with an
uppercase "B", the realm string must be enclosed in double (not single) quotes,
and exactly one space should precede the 401 code in the
HTTP/1.0 401 header line. Authentication parameters have
to be comma-separated as seen in the digest example above.

Instead of simply printing out PHP_AUTH_USER
and PHP_AUTH_PW, as done in the above example,
you may want to check the username and password for validity.
Perhaps by sending a query to a database, or by looking up the
user in a dbm file.

Watch out for buggy Internet Explorer browsers out there. They
seem very picky about the order of the headers. Sending the
WWW-Authenticate header before the
HTTP/1.0 401 header seems to do the trick
for now.

As of PHP 4.3.0, in order to prevent someone from writing a script which
reveals the password for a page that was authenticated through a
traditional external mechanism, the PHP_AUTH variables will not be
set if external authentication is enabled for that particular
page and safe mode is enabled. Regardless,
REMOTE_USER can be used
to identify the externally-authenticated user. So, you can use
$_SERVER['REMOTE_USER'].

Note:
Configuration Note
PHP uses the presence of an AuthType directive
to determine whether external authentication is in effect.

Note, however, that the above does not prevent someone who
controls a non-authenticated URL from stealing passwords from
authenticated URLs on the same server.

Both Netscape Navigator and Internet Explorer will clear the local browser
window's authentication cache for the realm upon receiving a
server response of 401. This can effectively "log out" a user,
forcing them to re-enter their username and password. Some people
use this to "time out" logins, or provide a "log-out" button.

This behavior is not required by the HTTP Basic authentication
standard, so you should never depend on this. Testing with Lynx
has shown that Lynx does not clear the authentication credentials
with a 401 server response, so pressing back and then forward
again will open the resource as long as the credential
requirements haven't changed. The user can press the
'_' key to clear their authentication information, however.

Also note that until PHP 4.3.3, HTTP Authentication did not work
using Microsoft's IIS server with the CGI version of PHP due to a
limitation of IIS. In order to get it to work in PHP 4.3.3+,
you must edit your IIS configuration "Directory Security". Click
on "Edit" and only check "Anonymous Access", all other fields
should be left unchecked.

Another limitation is if you're using the IIS module (ISAPI) and PHP 4, you
may not use the PHP_AUTH_* variables but instead, the
variable HTTP_AUTHORIZATION is available. For example,
consider the following code: list($user, $pw) = explode(':',
base64_decode(substr($_SERVER['HTTP_AUTHORIZATION'], 6)));

Note:
IIS Note:
For HTTP Authentication to work with IIS, the PHP directive
cgi.rfc2616_headers must
be set to 0 (the default value).

Note:
If safe mode is enabled, the
uid of the script is added to the realm part of
the WWW-Authenticate header.